How Facebook Figures Out Everyone You’ve Ever Met
Behind the Facebook profile you’ve built for yourself is another one, a shadow profile, built from the inboxes and smartphones of other Facebook users. Contact information you’ve never given the network gets associated with your account, making it easier for Facebook to more completely map your social connections.
Shadow contact information has been a known feature of Facebook for a few years now. But most users remain unaware of its reach and power. Because shadow-profile connections happen inside Facebook’s algorithmic black box, people can’t see how deep the data-mining of their lives truly is, until an uncanny recommendation pops up.
Facebook isn’t scanning the work email of the attorney above. But it likely has her work email address on file, even if she never gave it to Facebook herself. If anyone who has the lawyer’s address in their contacts has chosen to share it with Facebook, the company can link her to anyone else who has it, such as the defense counsel in one of her cases.
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Handing over address books is one of the first steps Facebook asks people to take when they initially sign up, so that they can “Find Friends.”
Update (2017-11-15): Christopher P. Atlan:
God damn Facebook. You need to press learn more to be able to skip the import of your contacts.
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It's worth mentioning that it's already been more than thirteen years since "Reason" magazine caused a big splash by sending each print subscriber an issue with a custom-printed cover that included a satellite photo of the subscriber's mailing address along with a significant amount of demographic data inside the cover, all of which was available to the publisher by combining subscriber info with info from publicly available databases. At the time, many people found it to be a shocking invasion of "privacy." It's interesting, but not surprising, how things have progressed/regressed in the intervening years.
Related: https://gizmodo.com/how-facebook-outs-sex-workers-1818861596